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Impedance transformer flags failed fuse

-December 17, 2004

Figure 1 depicts a circuit that detects the opening of a miniature circuit breaker or high-rupture-capability fuse in a high-reliability telecommunications power supply. The circuit generates an alarm when a failure changes the impedance of an electromagnetic sensor. Traditional fault-detection circuits sense the voltage difference developed across an open fuse, leakage current flowing through a fused circuit, or closure of an auxiliary (volts-free) contact by an actuator fuse. All three methods suffer from disadvantages: Voltage-difference circuits can introduce unacceptable delays as long as 30 minutes because the system's batteries sustain the bus voltage. Leakage-current sensors rely on the presence of a load that may not be present under certain conditions. Adding auxiliary miniature-circuit-breaker support circuits or special high-rupture-capability indicator fuses and their connectors can significantly increase system cost.

Capacitor C4 and the secondary inductance, L2, of transformer T1 resonate at approximately 42 kHz, a frequency that minimizes noise production in the audio, RF, and psophometric noise bands. Operational amplifier IC1 and associated components form an ac-coupled positive-feedback amplifier with a gain of 20. Under normal operation, an intact fuse or closed circuit breaker completes a low-impedance path through T1's single-turn primary (sense) winding. Transformer action presents a low impedance at the junction of C2, C4, and R5 and reduces the loop gain around IC1 to an amount insufficient to sustain oscillation.

When a fault occurs and interrupts current through T1's primary winding, its secondary impedance increases, allowing full loop gain and permitting IC1 to oscillate at 42 kHz, which L2 and C4 determine. Under fault conditions, T1's turns ratio injects less than 10 mV of wideband conducted noise into the dc bus. Capacitor C3 couples the oscillating signal to IC2, a gain-of-3 amplifier, which in turn drives a peak detector formed by D3 and C5. Transistor Q1 saturates and provides a logic-low signal to an external alarm. Figure 2 shows a typical application for sensing backup-battery-circuit failure.

To design transformer T1, you calculate the required impedance and turns ratio. Equation 1 describes the basic transformer relationship:

 EQUATION 1

where Z1 is the impedance of the primary winding, Z2 is the impedance of the secondary winding N1 is the number of primary turns, and N2 is the number of secondary turns.

Under normal operation with current flowing in the primary winding, the secondary impedance comprises the low primary-side impedance plus T1's leakage reactance. When no current flows in the primary winding, the number of turns in the secondary and the toroidal core AL (inductance per turn) determine the secondary winding L2's inductance and number of turns per Equation 2:

  EQUATION 2

where N2 is the number of turns around the toroidal core.

Ferrite-core manufacturers publish inductance-per-turndata that simplifies alteration of T1's design, but if that data is unavailable, you can use Equation 3 to calculate the inductance.

  EQUATION 3

where µe, the effective permeability, equals the magnetic constant, 4π×10–7Hm–1, I is the path length, and A is the cross-sectional area in millimeters squared.

Select a core that presents a high value of inductance to ensure that the difference between an open and a closed primary circuit causes a large change in relative secondary-winding impedance. Also, select a core material that doesn't saturate at full primary current.

Note that the core's central area must provide clearance for the battery cable (primary winding) and secondary winding. This application uses a Philips 3C85 toroidal ferrite core (part no. TN 16/9.6/6.3-3C85) with a secondary winding comprising five turns of 0.2-mm2 insulated copper wire. (Philips, however, has discontinued the 3C85 ferrite core. Ferroxcube's type 3C90 ferrite may serve as a replacement. Specifications are available at www.ferroxcube.com.) Figure 3 shows the completed transformer.

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